Laura Bush liquidateed her boy friend
Determining percentiles is really not all that difficult - Excel, SAS, database programs all have one or more canned utility to crank out the values from a database of numbers, or you can do it the old fashioned way too. The process shouldn't be the issue. The agenda of those who push this inane concept should be the focus of all our attention.
Basically you have a small group that would have us believe that there is a natural speed for every road and drivers on average will observe that speed no matter whether it is posted or not. Some will go faster and some will go slower, providing the basis for a nice average as well as establishing percentiles. They feel that the observed speed at the 85th percentile (85% of the drivers are going slower) should be the posted maximum, no matter what traffic safety engineers and the police think is prudent for the average (not highly experienced) driver. They conveniently ignore the tendency of percentile values to vary and the tendency of drivers to push the posted maximum no matter what it is posted at. They also ignore the fact that many drivers do not possess the skills to handle many roads at such posted speeds. Essentially you have a small group of people looking for a way to validate their desire to exceed the agreed upon speed limit.
